Recently, the operation speed and integration density of semiconductor devices have increased considerably, and the size of the semiconductor devices has been reduced accordingly. For this reason, there are demands to form a fine pattern exceeding a resolution limit of the conventional patterning apparatus using an optical system, that is, the resolution limit of an imaging optical system of a wafer stepper, for example.
First, a description will be given of the general operation of a conventional demagnification projection exposure apparatus, by referring to FIG. 1.
A light from a light source such as a mercury lamp is irradiated on a reticle 1. The reticle 1 has a reticle pattern in which a chromium (Cr) light blocking layer 3 is formed on a glass substrate 2, so as to expose a hole having a rectangular shape corresponding to the rectangular shape of the light blocking layer 3. The reticle pattern is reduced by a demagnification projection lens 4 and is imaged on a wafer 5 so as to expose a positive resist on the wafer 5.
FIG. 2 is a diagram for explaining the light intensity of the exposure light on the positive resist. FIG. 2(a) shows a partial cross section of the reticle 1. FIG. 2(b) shows the light amplitude distribution on the positive resist of the wafer 5 for the case where the exposure is made using the reticle 1. FIG. 2(c) shows the light intensity distribution on the positive resist of the wafer 5 for the case where the exposure is made using the reticle 1.
When the exposure is made by the light transmitted through the reticle 1 having the rectangular light blocking layer 3, the light intensity distribution on the positive resist of the wafer 5 has a negative peak having a relatively gradual slope. Hence, it is impossible to form a fine pattern which has a narrow width using such a light intensity distribution. In order to form a fine pattern which has a narrow width, it is necessary to make the slope of the negative peak sharper than that of the light intensity distribution shown in FIG. 2(c).
For example, if the exposure light is an i-line having a wavelength of 0.365 .mu.m and the numerical aperture of the optical system is 0.50, the resolution limit is approximately 0.4 .mu.m.
The slope of the negative peak of the light intensity distribution shown in FIG. 2(c) is dependent on the resolution of the imaging optical system. The resolution of the imaging optical system is determined by the exposure wavelength, the numerical aperture, the inconsistent performance of each individual lens itself and the like.
Accordingly, when the conventional demagnification projection exposure apparatus is used, it is impossible to form a fine hole which exceeds the resolution limit of the imaging optical system, and there is a problem in that the conventional demagnification projection exposure apparatus cannot cope with the patterns of the semiconductor devices which are becoming finer as the integration density is improved.
In order to eliminate the above described problem, the so-called phase shift method has been proposed. According to the phase shift method, the phase of the light transmitted through the reticle is shifted by a phase shift layer, so as to improve the resolution and contrast of the exposed image on the resist.
FIG. 3 shows the imaging optical system of the demagnification projection exposure apparatus and the reticle used when carrying out the phase shift method. In FIG. 3, those parts which are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted.
In FIG. 3, a phase shift reticle 6 is used in place of the reticle 1 shown in FIG. 1. The phase shift reticle 6 is made up of the glass substrate 2 and a phase shift layer 7 which is formed on the glass substrate 2.
FIG. 4 is a diagram for explaining the light intensity of the exposure light on the positive resist when the phase shift reticle 6 is used. FIG. 4(a) shows a partial cross section of the phase shift reticle 6. The pattern which is to be exposed is formed by an edge part of the phase shift layer 7 which is formed on the glass substrate 2 of the phase shift reticle 6. A transmitted light 8' which has passed through the glass substrate 2 and the phase shift layer 7 has a phase which is shifted by 180.degree. (.pi.) with respect to a transmitted light 8 which has passed through only the glass substrate 2.
FIG. 4(b) shows the light amplitude distribution on the resist of the wafer 5 when the exposure is made using the phase shift reticle 6. As shown, the light amplitude becomes zero at a position of the resist corresponding to the edge part of the phase shift layer 7, and the light amplitude sharply reverses on both sides of the edge part.
FIG. 4(c) shows the light intensity distribution on the resist of the wafer 5 when the exposure is made using the phase shift reticle 6. Because the light intensity is proportional to the square of the light amplitude, the light intensity sharply becomes zero at a position of the resist corresponding to the edge part of the phase shift layer 7. Accordingly, it is possible to form on the resist a fine line-and-space pattern which has satisfactory resolution and contrast.
An example of a fine pattern formed by the phase shift method will be described with reference to FIG. 5. FIG. 5(a) shows a plan view of the phase shift reticle 6 which has a square phase shift layer 7. FIG. 5(b) shows a fine resist pattern 10 which is formed by the edge part of the phase shift layer 7 of the phase shift reticle 6 shown in FIG. 5(a). As shown in FIG. 5(b), the fine resist pattern 10 is formed at the side part of the square, that is, along a part on the resist corresponding to the edge part of the phase shift layer 7.
On the other hand, the phase shift layer 7 may be arranged in a checker board pattern in the plan view as shown in FIG. 6(a). When the phase shift reticle 6 having the phase shift layer 7 shown in FIG. 6(a) is used for the exposure, it is possible to form a resist pattern 10 which has edges with a fine contrast over a relatively large area as shown in FIG. 6(b).
However, when the phase shift reticle 6 described above is used for the exposure, it is impossible to pattern a cut part indicated by a dotted line in FIG. 5(b). In other words, the edge part of the phase shift layer 7 inevitably takes a closed contour (or loop), and there is a problem in that the actual patterns of integrated circuits (ICs) cannot be formed using the phase shift reticle 6.
Accordingly, in order to overcome this problem of the phase shift method, a modified method has been previously proposed to form the resist pattern using a phase shift reticle which has two kinds of phase shift layers as shown in FIG. 7. FIG. 7(a) shows a plan view of the resist pattern which is to be formed.
FIG. 7(b) shows a plan view of the previously proposed phase shift reticle 6. This phase shift reticle 6 has the phase shift layer 7 for shifting the phase of the exposure light by 180.degree. (.pi.) at the edge part which is used for the pattern forming, and a phase shift layer 12 for shifting the phase of the exposure light by 90.degree. (.pi./2). This phase shift layer 12 is provided adjacent to the phase shift layer 7 at a part where no pattern forming is made, that is, at a part where the pattern is to be cut. A region indicated by hatchings in FIG. 7(b) is the region of the phase shift layer 7 which shifts the phase of the exposure light by 180.degree.. On the other hand, a region indicated by dots in FIG. 7(b) is the region of the phase shift layer 12 which shifts the phase of the exposure light by 90.degree.. Unmarked regions other than the hatched and dotted regions are the regions of the glass substrate 2.
By appropriately setting the exposure condition or the developing condition of the resist, only the edge part formed by the phase shift layer 7 and the glass substrate 2 as shown in FIG. 7(c). Other parts, that is, the edge part formed by the phase shift layers 7 and 12 and the edge part formed by the phase shift layer 12 and the glass substrate 2, are not patterned. The edge part formed by the phase shift layer 12 and the glass substrate 2 is indicated by a dotted line in FIG. 7(c).
However, this previously proposed modified phase shift method has a problem in that it is extremely difficult in actual practice to realize the phase shift reticle 6 having the phase shift layers 7 and 12 which are finely shaped as described. Accordingly, there are demands to realize a more feasible phase shift method which can form fine resist patterns and to realize a mask suitable for carrying out such a phase shift method.